1. Field of the Invention
The present invention relates to an automatic document feeder (ADF) and an image forming apparatus, for example, to an automatic document feeder which automatically feeds an original sheet by sheet from a bundle of originals placed on an original tray and an image forming apparatus including the automatic document feeder, such as a facsimile machine, a copying machine, or a multi-function peripheral (MFP).
2. Description of the Related Art
In general, in an image forming apparatus including an automatic document feeder, as a scanning method of scanning an image of an original, there are methods including a so-called book type scanning method in which an original on a glass platform is scanned while a traveling member is driven to move a scanning position and a so-called sheet-through type scanning method in which an image scanning unit is fixed and an original as a scanning target is fed at a constant rate.
In the sheet-through type scanning method, the original is automatically fed to the scanning position from a bundle of originals set in the automatic document feeder, and scanning is carried out. For this reason, the sheet-through type scanning method is more advantageous in increasing a productivity in scanning an original than the book type scanning method. In recent years, there is an attempt to increase a scanning rate of the original in seeking for further improvement in the productivity by using the sheet-through type scanning method.
In the related art, as an automatic document feeder which realizes a sheet-through type scanning method, it is known that there is a type of an automatic document feeder which includes a separating section on a downstream side of a paper feeding port. The separating section includes a conveying roller and a separator which is pressed against the conveying roller by a resilient member, and when an original being made of a plurality of sheets is fed from the paper feeding port, the original is separated sheet by sheet by frictional resistance generated between the conveying roller and the separator in the separating section to convey to the scanning position on the downstream side of the separating section (for example, see Japanese Patent Application Laid-open No. H10-095552).
In the automatic document feeder described in Japanese Patent Application Laid-open No. H10-095552, in scanning an original, such as a photograph original that requires high resolution, an engaging member serving as an operating portion is made to slide to forcibly separate the separator from the conveying roller so that a gap is formed between the conveying roller and the separator. Thus, in scanning an original which requires high resolution, it is possible to convey the original at a uniform conveying rate without applying a load to the original which passes through the separating section.
Incidentally, in the sheet-through type scanning method, it is necessary to scan an image with no bending, which is often called to be skewed. Hence, in the automatic document feeder, it is necessary to convey an original to a scanning position without being skewed.
As the automatic document feeder of the related art, for example, there is an automatic document feeder which is known to have a function to separate an original, when two sheets of the original are fed from a bundle of original that has been set in a paper feeding port as being overlapped (which is referred to as a double feeding hereafter), the double-fed original is separated into two sheets by a paper feeding belt and a reverse roller. In this automatic document feeder, during the above-described original separating operation, force in the conveying direction exerted to a first sheet of the original by the paper feeding belt conflicts with force in the direction opposite to the conveying direction exerted to a second sheet of the original by the reverse roller, so that the conveying process of the original is likely to go out of balance to cause a skew in the original. A direction of the conveying force of a conveying unit such as the conveying roller may deviate from a principal direction of scanning due to a variation in production accuracy of components included in the automatic document feeder. Therefore, there is a possibility that a skew may occur due to the variation of the direction in the conveying force from a principal direction of scanning even for an original which has passed through the separating section that includes the paper feeding belt and the reverse roller which are equipped to avoid an occurrence of the double feeding.
In recent years, an automatic document feeder is proposed in which a skew of an original is corrected so that an image without a skew is scanned. In this type of automatic document feeder, an abutting roller or the like for a skew correction is provided on the upstream side of the scanning position, and an original abuts on the abutting roller so as to correct a skew.
However, even when a skew correction has been carried out, if a variation occurs in the conveying force by the conveying unit in a conveying process that follow, the original after skew correction begins to be folded, and a skew may occur again before the original reaches the scanning position.
In the automatic document feeder of the related art, as an origin for an occurrence of a skew in the original, in addition to a deviation in the direction of the conveying force from the principal scanning direction of the conveying unit, it is exemplified that the separating section becomes a conveying resistance against the conveying force by the conveying unit.
In particular, on a so-called edge face basis in which one side surface of the original is fixed, depending on a size of the original, the conveying resistance in the separating section may become asymmetric with respect to a center of the original. This may cause an imbalance in the conveying force to trigger the occurrence of a skew. On the contrary, even on the so-called center basis in which movable side guides arranged on both sides to be symmetric with respect to the conveying center, in a plurality of pressing rollers or the like arranged in the width direction of the original, for example, if a variation occurs in the pressing balance in each component, similarly to the edge face basis, a skew may occur to the original, too. In the center basis, when side fences are not appropriately set in setting the original, such as a case where the side fences are set asymmetrically or a case where the side fences are set at positions so that gaps open between the originals and the side fences, the original is asymmetrically balanced with respect to the separating section, and similarly to the above, a skew of the original may occur.
In order to stably convey the original, the conveying force of the conveying roller on the downstream side of the separating section is generally set to a value so as to overcome the conveying resistance in the separating section. In this case, it is necessary to set the pressing force of the conveying roller on the downstream side of the separating section to be large and, hence, it is necessary to increase rigidity of members around the conveying roller.
In the meantime, in order to reduce the conveying resistance in the separating section, for example, as in the automatic document feeder described in Japanese Patent Application Laid-open No. H10-095552, it is considered that the separator is forcibly separated from the conveying roller to reduce the conveying resistance in the separating section.
In this case, as described above, in order to improve the productivity in scanning an original, it is necessary to shorten an interval between the sheets of the original to be conveyed, and thus it is necessary to carry out the separating operation in the separating section at a short interval. Thus, after the separating pressure in the separating section is released so as to reduce the conveying resistance, it is necessary to apply the separating pressure as soon as the next sheet of the original is ready to be fed.
However, in the automatic document feeder described in Japanese Patent Application Laid-open No. H10-095552, a separating section is designed such that separating pressure serving as conveying resistance in the separating section is simply released by a manual operation depending on the types of originals, and the separating pressure is not released and applied automatically while the original is being conveyed, for example. Thus, in the automatic document feeder described in Japanese Patent Application Laid-open No. H10-095552, no consideration is given to an application of the separating pressure after the separating pressure is released, so that no clue has been provided to suppress the occurrence of a skew in an original as well as to improve the productivity in canning the original.